Watch Dave & Julia's "Dreamgirl"

In what marks her first acting gig since giving birth to 8-month-old twins Hazel and Phinnaeus, the actress stars in a series of dream sequences, including an opening scene filmed at the Brooklyn Botanic Garden. Roberts, 37, worked for free, with one small request: breaks to nurse the babies.

Roberts had to take cover from the rain during the shoot at the botanical gardens, but "she has such a calm presence," says bassist Stefan Lessard, who plays her husband in the video. Added violinist Boyd Tinsley: "She's just the epitome of the American sweetheart."

In the video Roberts is trailed by a shadowy character who turns out to be Matthews. In real life, he said, "being pursued probably isn't unusual for Julia." (Between takes, Matthews kept his star happy by offering to fetch her favorite soy cappuccinos.)

In real life, Roberts says she's always been a fan of the group. "How long have the guys been together? Fourteen years? Okay, so I became a fan 13-1/2 years ago," she joked. "I'm the band mascot."

The respect is mutual for Matthews, who joked: "Hopefully it will spike record sales a bit by having Julia in it. No, that's not what it's about. It's really about art."